The increasing use of electronic data processing systems in industrial plants increasingly leads to the automation of plant-internal conveying processes. In this connection, material-flow systems are frequently controlled with the aid of code markings on individual conveying units of the system and reading devices for identifying the individual conveying units and for distributing the conveying units to the desired target positions. Whilst the selection of suitable optical, magnetic or mechanical coding marks presents no problems in most cases, the reading of the coding marks and the reliable recognition of the different bit patterns is problematic because of the continuous movement of the material-flow systems and the necessarily relatively short time available for the reading.
As a code, the so-called bar code is frequently used which can be automatically read with the aid of a laser scanner. Laser scanners having a moving scanning beam provide the possibility of multiple scanning of the coding marks which is important for the reliable recognition of the bit patterns. Although this is a relatively reliable reading method, experience shows that the error rate is relatively great, at least in the case of moving objects. In addition, for economic reasons it is not possible, as a rule, to equip conveying systems with laser scanners of this type because of the relatively high cost.
Considerably less expenditure is required if lasers with a fixed scanning beam are used which, however, must be passed several times to and fro over the code marking to achieve reliable recognition. This would also require an oscillating movement of the reading device, which presupposes a high constructional effort. In practice, it is impossible to achieve the necessary uniform sequence of motion within a specified time pattern at always the same distance from the code to be read. Problems would also be found with respect to control engineering since, with an oscillating sequence of motion, it must be possible to interpret the code both in the forward and in the reverse direction. A suggestion in the case of conveying systems is therefore to oscillate, not the reading device, but the individual conveying units with the code marking. However, this is disadvantageous in most cases because of the forces of mass inertia to be overcome.